No, you don’t understand.
Dean seeing “Dean” & “Sam” staying a little bit too close was like:
“they are brothers!”
But his reaction to “Dean” and “Cas” making out? Totally flawless.
He didn’t interrupt, he didn’t say a word. He was just like:
“well, okay. kinda cute. kinda wanna try it one day.”
And I think it’s beautiful.